Centrifugal pumps of conventional design have their components designed to operate at maximum efficiency at their design flow rate. When a centrifugal pump is required to deliver less than the design flow, the pump wastes considerable power, with this power being dissipated in large part as an increase in the temperature of the fluid being pumped and with potential for instability in operation.
Proposed solutions to the foregoing problems inherent in operation of a centrifugal pump at relatively low flow rates less than design flow rate are disclosed in the Davis U.S. Pat. No. 3,784,318 and a patent owned by the assignee of this application, namely, Caine U.S. Pat. No. 4,643,639.
The Davis patent discloses the use of a variable diffuser valve which can be positioned at any one of various selected positions to throttle the pump discharge by partial closing of diffuser entry passages adjacent the tips of the impeller blades. The Davis patent also describes a prior art structure using a diffuser inlet shutter. Servo means control the position of either the variable diffuser valve or the diffuser inlet shutter.
The Caine patent has a valve, in the form of a hollow cylinder 42, positionable by operation of an external actuator to control the position thereof relative to the entry end of a plurality of diffuser passages and with certain of the diffusers constructed to have portions of the passages remain open, even when the valve is seated on its seat.
The known prior art does not disclose a centrifugal pump having a self-regulating impeller discharge shutter automatically positionable in response to the pressure relation between the static pressure at the tips of the impeller blades and the pressure in the volute surrounding the impeller to throttle the flow delivered by the impeller to the volute.